The present invention relates to an X-ray permeable membrane for X-ray lithographic mask or, more particularly, to an X-ray permeable membrane for X-ray lithographic mask without defects or pinholes and having excellent smoothness of the surface and high transmissivity of visible light as well as excellent resistance against chemicals and moisture and high-energy beam irradiation.
In recent years, the X-ray lithographic method is highlighted for fine patterning of semiconductor devices in place of the conventional photolithographic method. The X-ray lithographic method is performed by using an X-ray lithographic mask made of a frame-supported X-ray permeable membrane, which is required to satisfy following requirements including, for example, that:
(1) the membrane is made from a material highly resistant and stable against irradiation with high-energy beams such as X-rays, high-energy electron beams, synchrotron orbital radiation, referred to as SOR hereinafter, and the like; PA0 (2) the membrane has a high transmissivity of visible light of at least 50% even when the membrane is thick enough to ensure good mechanical strengths so as to enable high-precision alignment; PA0 (3) the membrane is highly resistant against chemicals and moisture not to be affected or damaged in the manufacturing process of semiconductor devices using an etching solution, rinse and the like; PA0 (4) the membrane has a very smooth surface and is free from any defects such as pinholes; and so on.
Several kinds of materials have been proposed heretofore and are used as a material of the X-ray permeable membrane for X-ray lithographic mask including, for example, boron nitride BN, silicon nitride Si.sub.3 N.sub.4, silicon carbide SiC and the like. Each of these known materials has its own advantages and disadvantages so that no quite satisfactory material is available in respect of all of the above mentioned requirements for the material.
Among the above mentioned prior art materials, the most promising is silicon carbide and membranes of silicon carbide can be formed by the so-called chemical vapor-phase deposition method, referred to as the CVD method hereinafter, including thermal CVD method, plasma-induced CVD method and others, sputtering method and the like. A silicon carbide membrane formed by the thermal CVD method, in which decomposition of the reactants proceeds by means of thermal energy, has high crystallinity and is highly resistant against high-energy beam irradiation but has a disadvantage that the surface smoothness is very poor so that it can hardly be used as a membrane for the X-ray lithographic mask used in the manufacture of VLSIs which requires a patterning fineness of design rule 0.5 .mu.m or smaller unless the silicon carbide membrane as formed is subjected to improvement of the surface smoothness by taking a measure such as polishing, etching and the like resulting in a very high cost for the preparation of the X-ray lithographic mask.
Although the surface smoothness and transmissivity to visible light of a silicon carbide membrane can be improved when the membrane is prepared by utilizing the plasma CVD method for the deposition of a silicon carbide film on a substrate, such a silicon carbide membrane necessarily contains a large amount of hydrogen originating in the reactant compounds, for which almost all of known reactant compounds contain hydrogen more or less, which causes a fatal defect that the membrane is poorly resistant against high-energy beam irradiation which causes removal of the hydrogen atoms out of the membrane. Further, silicon carbide membranes prepared by the sputtering method have disadvantages that the transmissivity to visible light thereof is usually low to be about 40% or lower when the membrane has a thickness of 2 .mu.m and have an amorphous structure susceptible to crystallization so that strain of the membrane is readily caused by the irradiation with high-energy beams such as SOR although the membrane is absolutely free from hydrogen and has excellent smoothness of the surface.